1. Field of the Invention
The present invention is related to the field of airflow sensors. More particularly, the invention relates to a silicon-based mass airflow sensor arranged on a membrane. The sensor includes heating elements and a measuring element to determine the intensity and direction of flow of the fluid medium based on the transfer of heat into the medium.
2. Description of Related Art
In the automotive industry in particular, a need has arisen for mechanically rugged and reliable sensors which have an extremely fast response time. For example, in an electronic engine control system it is often necessary to provide rate data of mass airflow to determine the desired air-fuel ratio for target fuel efficiency and emission requirements.
Conventional mass airflow sensors are of several types: "hot-wire" sensors, conventional bi-directional airflow sensors, and "reverse flow" compensating airflow sensors. Conventional "hot wire" sensors are fabricated with a fine resistive wire such as platinum or tungsten supported in an air stream or wound on a ceramic bobbin, or as a thin film deposited on the bobbin. In operation, a known current flows through the wire to heat the resistive element to a predetermined temperature. When air flows across the elements, it alters the rate of heat transfer from the heated element, thereby causing a temperature change in the wire as well as a resulting change in resistance. Corresponding circuitry determines the voltage level required to maintain the predetermined temperature. This voltage level is then used to indicate the magnitude of airflow.
The hot-wire type of sensor has several limitations. In particular, due to the sensor's significant thermal mass, it has a relatively low response time which can overestimate true flow. In addition, due to its size, the overall mass airflow sensor is often more bulky than desired. Furthermore, the sensor does not indicate flow direction.
Conventional bi-directional airflow sensors typically comprise a central heating element and upstream and downstream temperature sensing elements. In operation, the upstream sensing element is cooled slightly more than the downstream sensing element under airflow conditions, and the temperature difference between the two sensing elements results in an electrical current difference between them. This current difference is then converted to a voltage difference which represents the magnitude of the airflow. The sign of the difference between the temperatures indicates flow direction.
The conventional bi-directional sensor has a number of disadvantages. For example, the corresponding readout circuitry requires several interconnections between the sensing elements and the remainder of the circuit. Each additional interconnection represents an incremental increase in the overall sensor cost, as well as a potential reliability issue.
Finally, conventional "reverse flow compensating" airflow sensors comprise two independent heating elements parallel to each other, transverse to the airflow. Independent electronic circuits maintain each heating element at a predetermined temperature. The voltage level required to maintain the desired temperature of the upstream heating element indicates the magnitude of airflow. For reverse flow, this voltage is suppressed. Thus, the output of the sensor in reverse flow is similar to that of a "half wave" rectifier.
Accordingly, there is a need for a mass airflow sensor having a fast response speed, high flow sensitivity, long term reliability, and the ability to detect the direction as well as the magnitude of the airflow.